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Ensuring safety during the operation of electrical installations and protection against the adverse effects of electricity are factors that determine the danger of electric shock. Causes of electrical injuries Factors that determine the risk of injury to a person electrically

For personnel working with electrical installations, the priority is to eliminate injuries. A feature of electric shock is considered to be the inability of people to remotely, visually, by smell or other signs to determine the threat. The use of special devices allows this to be done effectively, but not in all cases. Some dangers cannot be foreseen even by experienced specialists. To prevent injuries, special labor protection rules have been developed, with the observance of which the likelihood of injury is significantly reduced.

Thermal and mechanical damage results

Causes of injury from electric shock

Accidental touch, through inattention, to bare current-carrying elements of energized electrical installations. These can be bare wires, during the repair process, contacts of household or industrial equipment, on switches or sockets for lighting lamps.
During operation as a result of mechanical damage, parts of electrical installations can damage the insulating layer of current-carrying wires and be under dangerous voltage.
Often, the cause of electric shock is the approach on wet ground to a high-voltage power line wire that has fallen to the ground.
When approaching current-carrying elements with a voltage higher than 1000V, an electric shock can be through a breakdown of the air space.
The cause of the defeat is damp walls of buildings, structures, inside which wires with unreliable insulation and grounded elements of metal structures pass.
There are cases of injury as a result of poor organization of labor protection measures, with unauthorized connection to the power supply network, when repair personnel are working. Work without first checking the implementation of safety measures, as well as the absence of voltage, the presence of interlocks, warning posters and other elements that prevent injury.

Damaging factors

The duration of exposure to an electric current on the body, the magnitude of the lesion current, the area of \u200b\u200bcontact and many other factors determine the nature of the injury and the degree of exposure:

mechanical actions - delamination, breakage of tissues;
thermal - burns, destruction of the blood vessel structure (Fig. above);
electrolytic influence - the decay of organic matter in human flesh, including blood;
biological influence - disruption of the work of natural biocurrents, which causes involuntary, convulsive contraction of individual muscles.

Types of injury

There are the following main types of electric shock.

Electrical trauma

It is characterized by damage to individual organs, tissue fragments. These can be signs left by an electric discharge, metallization of the skin. The electric arc causes swelling of the surface of the eyeball, evaporation of the mucous membrane on it. Mechanical impact is possible, leading to injuries such as bruises, fractures.

Electric burn -this is the destruction of individual organs, areas of the skin as a result of the effect of a current or an electric arc on the tissues. Burns resulting from the action of an electric current can be of different types:

Electric burn upon contact of a wet (sweaty) body with current-carrying elements, liquids heat and boil on the surface and inside the tissues. This process depends on the resistance of the affected area and the strength of the current. The released heat energy causes burns. Such injuries occur on electrical installations with a power of up to 2 kW, causing first or second degree burns.
Electric arc burnis obtained on a part of the human body under the influence of the large thermal energy possessed by the arc (temperature up to 350 ̊С). Third and fourth degree burns occur in electrical installations with a voltage of 6-10 kW.

the skin is obtained with a short circuit (short circuit) or an arc discharge, when an electrical circuit with a large load is opened. As a result of the metal melting at high temperatures, it splashes onto the surface of the skin.

Molten metal splashing in the event of a short circuit

Small metal fragments from conductive contacts (copper, aluminum or steel) adhere to the skin and penetrate into the tissue, piercing and burning the skin. Such lesions take on a rough metal cover. Subsequently, on the affected area, the skin exfoliates along with foreign bodies, the wounds heal.

An example of electrical metallization of the skin

- the result of direct contact with live elements. The contours of their outlines represent the surface of the elements with which there was contact, usually a circle or an ellipse, from terminals and wires. The dimensions of the prints are up to 10 mm, the material of the conductive parts determines the color of the signs, they can turn out yellow from copper, brass, gray from steel and white from aluminum. The result determines the chemical, mechanical effect of the current. The tumor under these signs has no inflammation and heals quickly. With large areas of the lesion, there are numbness, loss of sensitivity.

Such signs can leave an electrical discharge.

Mechanical damage -the result of instant muscle contraction, the elements of the blood supply system, blood vessels, and the skin are torn. There are limb fractures, joint damage.
Electrophthalmia -the effect of high power ultraviolet radiation on the eyeballs. The resulting arc has a wide spectrum of light rays, including infrared, visible colors and ultraviolet. The latter burns the surface of the eyes.

Electric shock

The human nervous system instantly reacts to a strong external stimulus. There may be high blood pressure, impaired functioning of the blood supply, respiratory organs. There are several phases following an electrical shock:

exciting phase;
exhaustion and lethargy of the nervous system occurs, the victim remains conscious, but complete indifference to what is happening around him appears. Breathing weakens, the pulse rate increases, this can last up to 20 hours, then the heart stops, and the person dies.

Electric shock

Passing through human tissue, electricity causes convulsive, involuntary muscle contraction. The severity of injury depends on the strength of the current and the duration of contact with a conductive surface. Small currents cause slight itching and tingling, at 10-15 mA, uncontrolled convulsions occur.

A large current paralyzes the nervous system, the victim cannot independently free himself from contact with the current conductors, which prolongs the time of exposure to damaging factors. Currents of 20-25 mA / 50 Hz knock down the rhythm of the heartbeat, paralysis of the respiratory system leads to death.

A current of 50-80 mA creates fibrillation of the muscle tissue of the heart, the heart and blood flows stop. Currents greater than 100 mA definitely kill a person in 2-3 seconds of exposure to the body. It has been noticed that voltages up to 100V are not as dangerous with direct current as with alternating current, especially destructive with a frequency of 50 Hz, close to the heartbeat frequency, therefore, its effect instantly causes arrhythmia.

Currents at 20–100 Hz are the most dangerous. The likelihood of damage to internal tissues is less when the frequency increases.

Currents with a frequency of hundreds of kHz do not destroy internal organs; they can only cause burns on the surface of the body. Alternating and direct currents with a voltage of 500V have equally dangerous damaging factors. The voltage of 600V with direct current becomes more destructive for humans than with alternating current.

Electric shocks are divided according to the severity:

I - convulsive muscle contractions, while the person is fully conscious;
II - the victim is unconscious, the heart and respiratory organs are functioning;
III - the victim is unconscious, there are irregularities in the rhythm of the heart and malfunctions of the respiratory organs;
IV - breathing and blood circulation stops, death occurs (clinical).

Clinical death -there is no breathing, no heartbeats are heard, a person does not feel pain stimuli, wide pupils that do not respond to changes in light intensity. The transition to death is accompanied by a lack of oxygen supply to the structures of the brain.

The duration of the absence of oxygen in the brain is allowed for 4 minutes, for a maximum of 8 minutes, after which irreversible destructive consequences occur.

Cardiac arrest is caused by a sharp contraction of the muscles in the affected area, in the areas of passage, including the heart. Reflex contractions of the heart, when the current flows bypassing the heart muscles, create conditions for fibrillation and cardiac arrest. In these cases, the currents are called fibrillation currents, they interfere with breathing when flowing through the muscles of the chest, which are involved in the respiratory process.

With a short touch of conductive elements in a healthy person, cardiac arrest does not occur, the muscle contracts, with the loss of current, it relaxes, and the heart continues to function. When paralysis of the heart or respiration occurs, both cases are possible at the same time, the working capacity of the organs does not spontaneously recover, the heart needs urgent forced massage in conjunction with artificial respiration.

Paths of electric current through the body

The route largely determines the severity of the lesion; different organs have heterogeneous structures, the resistance of which is different.

Currents pass along routes with less resistance, with higher conductivity. The main conductors are large elements of the circulatory system. There is a lot of fluid in these vessels, and the blood has good conduction properties.

Most likely routes:

hand - through the chest area - the second hand;
left or right arm - through the body - legs;
head - through the neck - hands;
head - through the body - legs;
leg - through the groin area of \u200b\u200bthe trunk - the second leg.

An example of the route of electric current through the human body right arm - through the torso - legs

The most dangerous routes are:

hand - through the heart - leg;
on the head;
on the spinal cord.

Cases of death of the victim are not excluded when currents pass from one leg to the other or through the arm to the other hand.

The path of electric current through the human body from one hand to the other

The main dangerous case is considered to be the passage of current from the left hand to the legs, but according to the statistics of injuries, the largest percentage of fatalities occurs when the route through the right arm to the legs.

It is possible that the right hand is used more often during work, therefore, it is more often injured. The value of the current between the points where it flows depends on the voltage and tissues along its path with different resistance:

In \u003dU\ Rtwhere

Iп - defeat currents;
U is the voltage between the contact of the victim with the conductor and the point of exit of the current;
Rt - tissue resistance.

Rt of each person is different, this determines the skin, which can be wet, damaged, in these cases it will be less. The current will increase accordingly, the lesion will be more severe. The cornea of \u200b\u200bthe skin has the greatest resistance. When the surface is dry, the resistance on intact skin can be between 10 and 100 kΩ. Wet skin has a resistance of 1000 ohms, on damaged skin, with cuts, scuffs 500-800 ohms.

On internal tissues in the range of 300-500 ohms, practice shows that a voltage of 50-200V is already breaking through the stratum corneum. The breakdown voltage difference is determined by certain conditions:

stratum corneum thickness;
density of distribution and filling of blood vessels;
completeness of filling and distribution over the surface of the sweat glands.

Based on these conditions, the resistance in different areas is different.

The severity of the injury is influenced by the surrounding conditions, humid air. High temperatures increase conductivity.

Factors that determine the degree of electric shock

What's dangerous. Video

You can learn about the consequences of electric shock from the video below.

It is important to take into account the condition of the injured person, age, psychological characteristics. People with heart disease, during physical exertion sweat faster and more profusely, alcohol reduces the resistance of body tissues. All this must be known in order to take timely measures to prevent injuries, make work safer, and, if necessary, correctly determine the degree of injury and provide first aid.

The outcome of an electric shock depends on the following factors: the electrical resistance of the human body, the strength of the current flowing through the body, the time of exposure to the current, the path of current flow, the frequency and type of current, the individual characteristics of the human body, the conditions of the external (environmental) environment and other factors.

The amount of current flowing through the human body depends on the touch voltage Uand the resistance of the human body R.

The resistance of the human body, a nonlinear value, depending on many factors: the resistance of the skin and its condition; on the magnitude of the current and the applied voltage; on the duration of the current flow.

The upper stratum corneum of the skin has the greatest resistance. In a dry and uncontaminated state, it can be regarded as a dielectric: the specific resistance of the stratum corneum reaches 10 5 -10 6 Ohm m, which is thousands of times higher than the resistance of other layers of the skin.

The resistance of the human body with dry, clean and intact skin ranges from 1000 to 100,000 ohms, and the resistance of the layers of the body is only 500-700 ohms.

As a calculated value at an alternating current of industrial frequency, the resistance of the human body (R 4)taken equal to 1000 ohms. In real conditions, the resistance of the human body is a variable value and depends on a number of factors.

With an increase in the current passing through the human body, its resistance decreases, since this increases the heating of the skin and increases sweating. For the same reason, the R 4with an increase in the duration of the current flow. The higher the applied voltage, the more human current / h, the faster the human skin resistance decreases.

With an increase in voltage, the resistance of the skin decreases tenfold, and, consequently, the resistance of the body as a whole decreases; it approaches the resistance of the internal tissues of the body, that is, to its lowest value (300-500 ohms). This can be explained by electrical breakdown of the skin layer, which occurs at a voltage of 50-200 V.

Contamination of the skin with various substances, especially those that conduct electric current well (metal or coal dust, scale, etc.), reduces its resistance.

The main damaging factor of the electric current is the strength of the current passing through the human body. Small currents cause only unpleasant sensations. At currents greater than 10-15 mA, a person is not able to independently free himself from current-carrying parts and the action of the current becomes prolonged (non-releasing current).At a current equal to 20-25 mA (50 Hz), a person begins to experience difficulty breathing, which increases with increasing current. Under the action of such a current, suffocation occurs for several minutes. With prolonged exposure to currents of several tens of milliamperes and an action time of 15-20 s, respiratory paralysis and death can occur. Currents of 50-80 mA lead to cardiac fibrillation, which consists in the irregular contraction and relaxation of the muscle fibers of the heart, as a result of which blood circulation stops and the heart stops. The action of a current of 100 mA for 2-3 s will result in death (lethal current).

At low voltages (up to 100 V), direct current is about 3-4 times less dangerous than alternating current with a frequency of 50 Hz; at voltages of 400-500 V, their danger is comparable, and at higher voltages, direct current is even more dangerous than alternating current.

The most dangerous current is industrial frequency (20-100 Hz). A decrease in the danger of the action of a current on a living organism is noticeable at a frequency of 1000 Hz and above. High-frequency currents, starting from hundreds of kilohertz, cause only burns without affecting internal organs. This is due to the fact that such currents are not capable of causing excitation of nerve and muscle tissues.

The path of passage of electric current through the human body plays a significant role in the outcome of the lesion. The danger of electric shock increases greatly when it passes through vital organs: heart, lungs, brain. However, the reflex effect of the current on them also occurs with other paths of its passage, although the danger of injury in this case is sharply reduced. The most dangerous such paths include the head-to-hand and head-to-feet loops, and the least to the leg-to-leg loop. However, fatal injuries are known when the current passed along the path leg-leg or hand-arm.

A person's mental and physical condition also affects the severity of an electric shock. With diseases of the heart, thyroid gland, etc., a person is more severely affected at lower current values, since in this case the electrical resistance of the human body and the general resistance of the body to external stimuli decrease. It is noted, for example, that for women the threshold values \u200b\u200bof currents are about 1.5 times lower than for men. This is due to the weaker physical development of women. When using alcoholic beverages, the resistance of the human body decreases, the resistance of the human body and attention decrease. With collected attention, the body's resistance increases.

The outcome of an electric shock is influenced by environmental conditions (temperature, humidity) and the environment (presence of conductive dust, corrosive vapors and gases). High temperature and humidity increase the risk of electric shock. The lower the atmospheric pressure, the higher the risk of injury. Dampness, corrosive vapors and gases have a destructive effect on the insulation of electrical installations.

Electrical installations are classified by voltage: with a rated voltage of up to 1000 V and over 1000 V. The safety of maintenance of electrical equipment also depends on environmental factors.

Depending on the presence of conditions that increase the risk of exposure to a person, all rooms are divided into the following classes for the risk of electric shock to people:

The first is premises without increased danger, in which there are no conditions that create an increased and special danger;

The second - premises with increased danger, characterized by the presence in them of at least one of the following signs: dampness (the relative humidity of the air for a long time exceeds 75%); high temperature (above + 35 ° С); conductive dust; conductive floors; the possibility of simultaneous contact of a person to the metal structures of buildings that have connections to the ground, on the one hand, and the metal cases of electrical equipment, on the other;

The third - especially dangerous premises, characterized by the following features: relative air humidity close to 100% (visually determined by the presence of condensation on the inner surface of the building structures of buildings and premises); chemically aggressive environment; the presence of two or more signs of premises with increased danger at the same time; as well as the territory for the location of outdoor electrical installations. According to the method of protecting a person from electric shock, electrical products are divided into five classes: 0, 01.1, II, III.

Class 0 includes products with a rated voltage of more than 42 V with working insulation and not having grounding devices. Household appliances are manufactured in class 0, as they are designed to work in rooms without increased danger.

Class 01 includes products with working insulation, grounding element. The wire for connecting to the power source does not have a grounding conductor.

Class I includes products with working insulation, a grounding element and a power supply conductor with a grounding conductor and a grounding type plug.

Class P includes products that have double or reinforced insulation for all parts accessible to touch with respect to parts that are normally energized, and do not have grounding elements.

Class III represents products without internal and external electrical circuits with a voltage not exceeding 42 V.

1. Amount of current - the main factor characterizing the severity of electrical injury. IN Annex M information about the influence of currents of different magnitude on the human body is given. Threshold values \u200b\u200bare used to characterize this effect:

- sensitivity threshold - the minimum current that a person feels. It is 0.6 ... 1.5 mA for AC (frequency 50 Hz) and 5 ... 7 mA for DC. Such a current is safe for humans;

- threshold non-release current - the minimum current strength at which a person cannot independently tear off his hands from live parts. In terms of magnitude, such a current is not dangerous to humans, but with prolonged exposure it can lead to serious consequences and even death. With direct current, a person can independently tear his hand away from the conductor at any current strength, however, at the moment of separation, painful muscle contractions occur, similar to those that occur with alternating current. A person is able to withstand pain when separated from live parts with a current strength of no more than 50 - 80 mA.

- threshold fibrillation current - the minimum current strength at which fibrillation of the victim's cardiac activity occurs. Causes death of the victim if the current travel time exceeds 1 s, is 100 mA for alternating current at 50 Hz and 300 mA for direct current. A current of more than 5 A causes immediate cardiac arrest, bypassing the state of fibrillation.

- maximum permissible current - the maximum amperage that does not cause electrical injury for any duration of action.

2. Kind and frequency of current - the resistance of the human body has a capacitive component, therefore, a change in the frequency of the applied voltage leads to a change in the total resistance of the body and an increase in the strength of the passing current.

An increase in the frequency of the current from 0 to 200 Hz leads to an increase in the risk of injury. At a current frequency of 100 kHz and above, there is only a risk of burns. Increasing the frequency further reduces the risk of AC shock, which disappears altogether at 450 kHz. At voltages up to 500 V, direct current is safer (4-5 times), above 500 V - direct current is more dangerous. The most dangerous for humans is an alternating current with a frequency of 50 Hz at a voltage of 220 V. Approximate values \u200b\u200bof the limiting values \u200b\u200bfor such a current are given in table. 6.1

Table 6.1. Threshold values \u200b\u200bof alternating current of frequency 50 Hz

3. Electrical resistance of the human body is determined by the resistance of the stratum corneum and depends on the applied voltage. Dry, intact skin has a resistance of 500 ... 500,000 ohms. Wet, contaminated skin has significantly less resistance, which is due to the passage of current through the sweat glands and the subcutaneous area. The resistance of the human body to an alternating current of 50 Hz is taken equal to 1,000 ohms.

A living organism consists of various cells and salt solutions, which causes different electrical resistance of different parts of the body. In addition, the resistance of the skin in different places of the human body is very different, so the severity of electrical injury depends not least on the site of the injury. Attention factor increases the resistance of the human body and reduces the likelihood of injury. It is known that about 85% of electrical injuries occur at the end of a work shift due to weakening of the attention of workers.

4. Duration of the current - with the passage of current, the resistance of the skin sharply decreases, which leads to more severe electrical injuries: after 30 s, the resistance of the body decreases by 25%, and after 90 s - by 70%. Table 6.2 shows the dependence of the maximum permissible current strength on the duration of its action.

Table 6.2 - Maximum permissible current strength (~ 50 Hz)

In addition, the body accumulates the effects of current exposure and increases the likelihood of coincidence of the moment of current passage with the vulnerable T-phase of the cardiac cycle (with a period of 0.15-0.20 s, during which the contraction of the ventricles of the heart ends and they pass into a relaxed state) ... That is why, when providing assistance, first, you need to stop the action of the current.

5. Direction of current flow - if vital organs (heart, lungs, brain) are in the path of the current, then the danger of injury is very great. With other directions of current flow, the severity of the lesion is significantly reduced. In practice, there are 15 possible paths for the passage of current in the human body, the most common of which are the directions "hand - arm" (40% of cases) and "right arm - legs" (20% of cases). The most dangerous paths are "head - arms" and "head - feet", which are rarely implemented in practice. The least dangerous path is the leg-to-leg path (lower loop), which occurs when a person is exposed to the tension of a step.

6. Connection diagram into an electrical circuit - a person can simultaneously touch two phase wires of the AC network (two-phase touch), one phase wire (single-phase touch), approach a dangerous distance to uninsulated live parts, touch the body of electrical equipment that is energized or enter the zone step voltage action.

5. Individual properties of a person - Physically healthy people are more likely to endure electric shocks than sick and weak people. The least resistant to the action of electric current are people with nervous diseases, diseases of the skin, cardiovascular system, organs of internal secretion, lungs. Physical and emotional stress increases the risk of electric shock to a person.

The nature and consequences of exposure to an electric current depend on the following factors: electrical resistance of the human body; voltage and current values; duration of exposure to electric current; paths of current through the human body; kind and frequency of electric current; environmental conditions.

The electrical resistance of the human body.The human body is a conductor of electric current, however, non-uniform in electrical resistance. The greatest resistance to electric current is provided by the skin, therefore the resistance of the human body is determined mainly by the resistance of the skin.

The skin consists of two main layers: the outer layer is the epidermis and the inner layer is the dermis. Outer layer - the epidermis, in turn, has several layers, of which the thickest upper layer is called the stratum corneum. The stratum corneum in a dry and uncontaminated state can be regarded as a dielectric: its specific volume resistance reaches 10 5 -10 6 Ohm-m, ie, thousands of times higher than the resistance of other layers of the skin and internal tissues of the body. Resistance inner layer of the skin- dermis - insignificantly: it is many times less than the resistance of the stratum corneum.

The resistance of the human body with dry, clean and intact skin (measured at a voltage of 15 - 20 V) ranges from 3 to 100 kOhm or more, and the resistance of the inner layers of the body is only 300 - 500 Ohm.

The internal resistance of the body is considered active. Its value depends on the area of \u200b\u200bthe body part through which the current flows.

The external resistance of the body consists, as it were, of two parallel-connected resistances: active and capacitive. In practice, capacitive resistance is usually neglected, which is insignificant, and the resistance of the human body is considered purely active and unchanged.

As a calculated value with an alternating current of industrial frequency, the active resistance of the human body equal to 1000 Ohm is used.

In actual conditions, the resistance of the human body is not constant. It depends on a number of factors, including the condition of the skin, the state of the environment, the parameters of the electrical circuit, etc.

Damage to the stratum corneum (cuts, scratches, abrasions, etc.) reduces the body's resistance to 500-700 ohms, which increases the risk of electric shock to a person.

Moisturizing the skin with water or sweat has the same effect. Therefore, working with electrical installations with wet hands or in conditions that cause skin moisture, as well as at elevated temperatures that cause increased sweating, exacerbate the risk of electric shock to a person.

Contamination of the skin with harmful substances that conduct electric current well (dust, scale, etc.) leads to a decrease in its resistance.

The body resistance is influenced by the area of \u200b\u200bcontacts, as well as the place of contact, since the skin resistance of the same person is not the same in different parts of the body. The least resistance is possessed by the skin of the face, neck, arms in the area above the palms and especially on the side facing the body, armpits, the back of the hand, etc. The skin of the palms and soles has a resistance that is many times greater than the resistance of the skin of other parts of the body.

With an increase in the current and the time of its passage, the resistance of the human body decreases, since this increases the local heating of the skin, which leads to the expansion of its vessels, to an increase in the supply of this area with blood and an increase in sweating.

With an increase in the voltage applied to the human body, the skin resistance decreases tenfold, approaching the resistance of internal tissues (300-500 ohms). This is due to the electrical breakdown of the stratum corneum, an increase in the current passing through the skin.

With an increase in the frequency of the current, the resistance of the body will decrease and at 10-20 kHz the outer layer of the skin practically loses its resistance to the electric current.

The magnitude of the current and voltage.The main factor determining the outcome of an electric shock is the strength of the current passing through the human body.

The voltage applied to the human body also affects the outcome of the lesion, since it determines the value of the current passing through the person.

Table 1. Threshold limits of currents of various magnitudes

Kind and frequency of electric current.Direct current is about 4-5 times safer than alternating current. This follows from the comparison of the threshold perceptible and non-releasing currents for direct and alternating currents. This position is valid only for voltages up to 250 - 300 V. At higher voltages, direct current is more dangerous than alternating current (with a frequency of 50 Hz).

For alternating current, its frequency also plays a role. With an increase in the frequency of the alternating current, the impedance of the body decreases, which leads to an increase in the current passing through the person, and therefore the risk of injury increases.

The greatest danger is the current with a frequency of 50 to 1000 Hz; with a further increase in frequency, the risk of injury decreases and completely disappears at a frequency of 45 - 50 kHz. These currents keep the risk of burns. The decrease in the risk of electric shock with increasing frequency becomes practically noticeable at 1 - 2 kHz.

Duration of exposure to electric current.The duration of the passage of the current through the human body has a significant effect on the outcome of the lesion. Prolonged exposure to current leads to severe and sometimes fatal injuries.

The influence of the duration of the passage of current through the human body on the outcome of the lesion can be estimated by the empirical formula:

I h \u003d 50 / t,

where I h - current passing through the human body, mA; t -duration of current passage, s.

This formula is valid in the range of 0.1-1.0 s. It is used to determine the maximum permissible currents passing through a person along the path of an arm and a leg, which are necessary for calculating protective devices.

For long-term exposure, the permissible safe current is assumed to be 1 mA.

With a duration of exposure up to 30 s -b mA.

When exposed to 1 s or less, the values \u200b\u200bof the currents are given below, however, they cannot be considered as ensuring complete safety and are accepted as practically permissible with a rather low probability of defeat:

These currents are considered permissible for the most probable paths of their flow in the human body: arm - arm, arm - legs and leg - leg.

Safe current values \u200b\u200bfor a given path of its flow and duration of exposure in accordance with GOST 12.1.038 - 82 are guided in the design, calculation and operational control of protective systems

The path of the current through the human body.The path of the current passing through the human body plays an essential role in the outcome of the lesion, since the current can pass through the vital organs: heart, lungs, brain, etc. The influence of the current path on the outcome of the lesion is also determined by the resistance of the skin in different parts of the body.

Possible pathways of current in the human body, which are also called current loops,enough. The most common current loops are: arm - arm, arm - legs, and leg - leg (Table 15.1).

The most dangerous loops are head-arms and head-legs, but these loops are relatively rare.

Table 15.1. Characterization of current paths in the human body

Individual properties of a person.It has been found that physically healthy and strong people tolerate electric shocks more easily.

Individuals suffering from diseases of the skin, cardiovascular system, organs of internal secretion, lungs, nervous diseases, etc. are distinguished by an increased susceptibility to electric current.

Safety regulations for the operation of electrical installations provide for the selection of personnel to service existing electrical installations for health reasons. For this purpose, a medical examination of persons is carried out upon admission to work and periodically once every two years in accordance with the list of diseases and disorders that prevent admission to service of existing electrical installations.

Environmental conditions.The condition of the surrounding air as well as the surrounding environment can significantly affect the risk of electric shock.

Dampness, conductive dust, corrosive vapors and gases that have a destructive effect on the insulation of electrical installations, as well as high ambient temperatures, lower the electrical resistance of the human body, which further increases the risk of electric shock.

The impact of current on a person is also aggravated by conductive floors and metal structures close to electrical equipment that have a connection to the ground, since in the case of simultaneous contact with these objects and the body of electrical equipment, accidentally energized, a high current will pass through the person.

Depending on the presence of the listed conditions that increase the risk of exposure to a person, the "Electrical Installation Rules" divide all premises according to the risk of electric shock to people into the following classes: no increased danger, with increased danger, especially dangerous, as well as the area where outdoor electrical installations are located.

1. Premises without increased dangercharacterized by the absence of conditions that create an increased or special danger (items 2 and 3).

2. Premises with increased dangercharacterized by the presence in them of one of the following conditions that create an increased danger:

a) dampness (relative humidity of air for a long time exceeds 75%) or conductive dust;

b) conductive floors (metal, earth, reinforced concrete, brick, etc.);

c) high temperature (above + 35 ° С);

d) the possibility of simultaneous contact of a person to the metal structures of buildings, technological devices, mechanisms, etc., having connections to the ground, on the one hand, and to the metal cases of electrical equipment, on the other.

3. Particularly dangerous premisescharacterized by the presence of one of the following conditions that create a particular hazard:

a) special dampness (the relative humidity of the air is close to 100%: the ceiling, walls, floor and objects in the room are covered with moisture);

b) chemically active or organic environment (destroying insulation and live parts of electrical equipment);

c) simultaneously two or more conditions of increased danger (clause 2).

4. Territories for the location of outdoor electrical installations.In terms of the danger of electric shock to people, these territories are equated to especially dangerous premises.

In the chemical industry, many production facilities are particularly hazardous.

Electrical equipment should be selected taking into account the state of the environment and the class of the premises for the risk of electric shock, in order to ensure the necessary degree of safety during its maintenance.

So. for example, electrical equipment installed in damp, especially damp and dusty rooms, as well as in rooms with a chemically active environment, must be of a closed type and have an appropriate design: drop or splash proof, dust proof, blown, etc.

Electrical equipment and electrical networks located in rooms with a chemically active environment must be selected taking into account the appropriate design or coating that protects them from the effects of this environment. When choosing places for laying electrical networks and methods of protecting them from corrosion, one should take into account the properties of the environment.

To protect electrical equipment from the effects of a chemically active environment, it is necessary that it comply with the operating conditions; the material from which the electrical equipment is made must be corrosion-resistant; metal parts must be reliably protected by paint and varnish or galvanized coating.

In conditions of exposure to chemically active media, chemical-resistant electrical equipment should be used.

In hazardous areas of all classes with chemically active environments, wires and cables with polyvinyl chloride insulation, as well as wires with rubber insulation and cables with rubber and paper insulation in a lead or PVC sheath must be used. The use of wires and cables with polyethylene insulation with any sheaths and covers is prohibited.

To ensure reliable operation of electrical equipment in chemically active environments, it is necessary to exclude the possibility of penetration of chemically active reagents into the shells of electrical equipment and to use special construction materials and protective coatings. The design of the input devices of electrical equipment must ensure the protection of live parts, insulation and joints from the effects of chemically active environments for which it is intended.

The amount of current flowing through the human body depends on the touch voltage Uand the resistance of the human body R.

The upper stratum corneum of the skin has the greatest resistance. In a dry and uncontaminated state, it can be regarded as a dielectric: the specific resistance of the stratum corneum reaches 10 5 - 10 6 Ohm * m, which is thousands of times higher than the resistance of other layers of the skin.

The resistance of the human body with dry, clean and intact skin ranges from 1000 to 100,000 ohms, and the resistance of the layers of the body is only 500-700 ohms.

As a calculated value at an alternating current of industrial frequency, the resistance of the human body (R 4 ) taken equal to 1000 ohms. In real conditions, the resistance of the human body is a variable value and depends on a number of factors.

With an increase in the current passing through the human body, its resistance decreases, since this increases the heating of the skin and increases sweating. For the same reason, the R 4 with an increase in the duration of the current flow. The higher the applied voltage, the more human current / h, the faster the human skin resistance decreases.

With an increase in voltage, the resistance of the skin decreases tenfold, and, consequently, the resistance of the body as a whole decreases; it approaches the resistance of the internal tissues of the body, i.e., to its lowest value (300-500 ohms). This can be explained by electrical breakdown of the skin layer, which occurs at a voltage of 50-200 V.

Contamination of the skin with various substances, especially those that conduct electricity well (metal or coal dust, scale, etc.), reduces its resistance.

The main damaging factor of the electric current is the strength of the current passing through the human body. Small currents cause only unpleasant sensations. At currents greater than 10-15 mA, a person is not able to independently get rid of live parts and the action of the current becomes long-lasting (non-releasing current).At a current equal to 20-25 mA (50 Hz), a person begins to experience difficulty breathing, which increases with increasing current. Under the action of such a current, suffocation occurs for several minutes. With prolonged exposure to currents of several tens of milliamperes and an action time of 15--20 s, respiratory paralysis and death can occur. Currents of 50-80 mA lead to cardiac fibrillation, which consists in the irregular contraction and relaxation of the muscle fibers of the heart, as a result of which blood circulation stops and the heart stops. The action of a current of 100 mA for 2 to 3 s will result in death (lethal current).

An essential role in the outcome of the lesion is played by the path of passage of electric current through the human body. The danger of electric shock increases greatly when it passes through vital organs: heart, lungs, brain. However, the reflex effect of the current on them also occurs with other paths of its passage, although the danger of injury in this case is sharply reduced. The most dangerous such paths include the head-to-hand and head-to-feet loops, and the least to the leg-to-leg loop. However, fatal injuries are known when the current passed along the path leg-leg or hand-arm.

A person's mental and physical condition also affects the severity of an electric shock. With diseases of the heart, thyroid gland, etc., a person is more severely affected at lower current values, since in this case the electrical resistance of the human body and the general resistance of the body to external stimuli decrease. It is noted, for example, that for women the threshold values \u200b\u200bof currents are about 1.5 times lower than for men. This is due to the weaker physical development of women. With the use of alcoholic beverages, the resistance of the human body decreases, the resistance of the human body and attention decrease. With collected attention, the body's resistance increases.

Electrical installations are classified by voltage: with a rated voltage of up to 1000 V and over 1000 V. The safety of maintenance of electrical equipment also depends on environmental factors.

Depending on the presence of conditions that increase the risk of exposure to a person, all rooms are divided into the following classes for the risk of electric shock to people:

* the first - premises without increased danger, in which there are no conditions that create an increased and special danger;
* the second - rooms with increased danger, characterized by the presence in them of at least one of the following signs: dampness (relative humidity for a long time exceeds 75%); high temperature (above + 35 ° С); conductive dust; conductive floors; the possibility of simultaneous contact of a person to the metal structures of buildings that have connections to the ground, on the one hand, and the metal cases of electrical equipment, on the other;
* the third - especially dangerous premises, characterized by the following features: relative air humidity close to 100% (visually determined by the presence of condensation on the inner surface of the building structures of buildings and premises); chemically aggressive environment; the presence of two or more signs of premises with increased danger at the same time; as well as the territory for the location of outdoor electrical installations. According to the method of protecting a person from electric shock, electrical products are divided into five classes: 0, 01.1, II, III.

Class 01 includes products with working insulation, earthing element. The wire for connecting to the power source does not have a grounding conductor.

Class I includes products with working insulation, a grounding element and a power supply conductor with a grounding conductor and a grounding type plug.

Class P includes products that have double or reinforced insulation for all parts accessible to touch with respect to normally energized parts and do not have grounding elements.

Class III represents products without internal and external electrical circuits with a voltage not exceeding 42 V.

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